Sharks...

How dangerous are sharks? Worldwide, approximately 12 people are killed by shark attacks each year. Meanwhile, a report published in December 2012 estimated that 100 million sharks are killed by humans every year. That translates to 11,408 sharks killed every hour. Sharks are mostly killed for their meat, fins, liver oil—or just for sport. Thousands of sharks are also killed each year as “bycatch”: by getting caught accidentally by fishing nets meant for other types of fish. Sharks have been around for 420 million years and survived every major mass extinction—including the one that wiped out the dinosaurs. But in the past few decades, shark populations have declined by as much as 90%. Today, many shark populations are on the brink of extinction; of the world’s 465 known shark species, at least 141 are near-threatened, vulnerable, endangered, or critically endangered. Among the 11 listed as critically endangered, at least 3 species have not been sighted for years and may already be extinct. If you are afraid of sharks, you might think this is a good thing. But sharks play a vital role in ocean ecosystems. Sharks are apex predators. This means that they are at the top of the food chain and have no natural predators. Most sharks feed on fish and invertebrates; and some larger shark species prey on seals, sea lions, smaller sharks, and other marine mammals. When shark populations decline, populations of their prey increase. This can lead to destructive imbalances in ocean food webs.

Take for example a study carried out on tiger shark populations in Hawaii. One of the tiger sharks’ main food sources was sea turtles. The sea turtles graze on sea grass, which is vital habitat for all the creatures in the area, providing both food and shelter. As tiger shark populations decreased from overfishing, turtle populations increased. More turtles meant more grazing. Additionally, in the absence of tiger sharks, turtles were able to spend all their time grazing in the same areas, because they were never forced to move in order to escape being eaten. As a result, the seagrass in the area was soon destroyed; and the entire ecosystem that relied on the sea grass for food and shelter collapsed. This is one example of how declining shark populations can create a domino effect of unintended consequences.

This same pattern can be observed whenever populations of apex species decline (wolves are another classic example). Though it may at first seem surprising, ecosystems rely on predators for survival. In the past couple of decades, as more scientists have studied sharks, more and more people have begun to recognize how important sharks are to the health of our oceans. Many groups are working to protect them. Especially in the US, more laws and policies are put in place every year: to limit the number of sharks that may be hunted legally, and to prevent unintentional shark deaths through bycatch. These actions have made a significant impact; a number of studies recently carried out on shark populations in US waters show that many shark populations are on the rise again. Though the same can’t be said in many other parts of the world, this shows that the more we understand ecosystems and use those findings to make laws and policy, the better our chances are of saving these ecosystems.